Method of polishing a substrate

ABSTRACT

A method of polishing a substrate by providing a polishing slurry comprising water and silica particles, wherein the average size (by number) of the silica particles is less than 30 nm, providing a polymeric polishing pad substantially free of bound abrasive particles and having a polishing surface comprising a multiplicity of cavities, and polishing the surface of the substrate by contacting the polishing slurry and the polishing pad with the substrate and moving the polishing pad relative to the substrate.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention pertains to a method of polishing a substrate, in particular a method of polishing a semiconductor device.

BACKGROUND OF THE INVENTION

[0002] In the production of semiconductor devices, it is often necessary to achieve a high degree of planarity in the surface of the device materials, for example, to allow the formation of inter-level dielectrics, while minimizing the amount of field dielectric loss and number of surface defects caused by the planarization process. As devices become smaller, the planarity, field loss, and defectivity of the device surfaces become increasingly important. One accepted method of planarizing semiconductor devices is through polishing the surface of the semiconductor with a polishing composition and/or a polishing pad. Various pads, polishing compositions, and methods of polishing semiconductor devices are known. For example, U.S. Pat. No. 5,575,837 discloses a polishing composition containing silica particles 30 nm and larger in size and a persulfate or hydrazine compound. U.S. Pat. No. 5,489,233 discloses the use of polishing pads having a surface texture or pattern. U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad. However, there continues to be a need for more efficient and more effective planarization and polishing methods. The present invention provides such a method. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

[0003] This invention provides a method of polishing a substrate, which method comprises providing a polishing slurry comprising water and silica particles, wherein the average size (by number) of the silica particles is less than 30 nm, providing a polymeric polishing pad substantially free of bound abrasive particles and having a polishing surface comprising a multiplicity of cavities, and polishing the surface of the substrate by contacting the polishing slurry and the polishing pad with the substrate and moving the polishing pad relative to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1A-1D are schematic views of a semiconductor device before the formation of a trench or opening (FIG. 1A), after the formation of a trench or opening (FIG. 1B), after the deposition of a dielectric layer (FIG. 1C), and after planarization of the dielectric layer (FIG. 1D) in the stages of a typical shallow trench isolation procedure.

[0005]FIGS. 2A and 2B depict a typical semiconductor device comprising an interlayer dielectric before planarization (FIG. 2A) and after planarization (FIG. 2B).

[0006]FIG. 3 is a graph of step height remaining (Å) versus polish time (sec.) with respect to the use of the method of the present invention (involving 20 nm colloidal silica polishing slurry) in conjunction with a grooved pad and a perforated pad to planarize the dielectric layer of a semiconductor device (with 52% pattern density).

DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention provides a method of polishing a substrate, which method comprises abrading the surface of a substrate with a polishing slurry and polishing pad. For the purposes of the present invention, the terms “polish” and “planarize” are used interchangeably to refer to abrasion of the surface of a substrate.

[0008] The Substrate

[0009] The present method of polishing a substrate can be used to polish or planarize any substrate, for example, a substrate comprising a metal, metal oxide, metal composite, semiconductor base material, or mixture thereof. The substrate can comprise, consist essentially of, or consist of any suitable metal. Suitable metals include, for example, copper, aluminum, tantalum, titanium, tungsten, gold, platinum, iridium, ruthenium, and combinations (e.g., alloys or mixtures) thereof. Preferably, the metal of the substrate is tungsten. The substrate also can comprise, consist essentially of, or consist of any suitable metal oxide. Suitable metal oxides include, for example, alumina, silica, titania, ceria, zirconia, germania, magnesia, and combinations thereof. Preferably, the metal oxide of the substrate is silica. In addition, the substrate can comprise, consist essentially of, or consist of any suitable metal composite. Suitable metal composites include, for example, metal nitrides (e.g., tantalum nitride, titanium nitride, and tungsten nitride), metal carbides (e.g., silicon carbide and tungsten carbide), nickel-phosphorus, alumino-borosilicate, borosilicate glass, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), silicon/germanium alloys, and silicon/germanium/carbon alloys. The substrate also can comprise, consist essentially of, or consist of any suitable semiconductor base material. Suitable semiconductor base materials include single-crystal silicon, poly-crystalline silicon, amorphous silicon, silicon-on-insulator, and gallium arsenide.

[0010] The present method is especially useful in polishing or planarizing a semiconductor device, for example, semiconductor devices having device feature geometries of about 0.25 μm or smaller (e.g., 0.18 μm or smaller). The term “device feature” as used herein refers to a single-function component, such as a transistor, resistor, capacitor, integrated circuit, or the like. As device features of the semiconductor substrate become increasingly small, the degree of planarization becomes more critical. A surface of a semiconductor device is considered to be sufficiently planar when the dimensions of the smallest device features (e.g., device features of 0.25 μm or smaller, such as device features of 0.18 μm or smaller) can be resolved upon the surface via photolithography. The planarity of the substrate surface also can be expressed as a measure of the distance between the topographically highest and lowest points on the surface. In the context of semiconductor substrates, the distance between the high and low points on the surface desirably is less than about 2000 Å, preferably less than about 1500 Å, more preferably less than about 500 Å, and most preferably less than about 100 Å.

[0011] The present method can be used in the manufacture of a semiconductor device, for example, in the formation of isolation structures by shallow trench isolation methods (STI) during the fabrication of a semiconductor device. As illustrated in FIG. 1A-1D, STI involves forming a trench or opening (i.e., via) (12) in the surface of the semiconductor base (10), which base is formed from any appropriate semiconductor material, such as single crystal silicon, gallium arsenide, silicon-on-insulator, or other suitable semiconductor materials known in the art. The base material (10) is preferably coated with a hard mask (11) of suitable material such as silicon nitride, tantalum nitride, or titanium nitride, prior to forming the trench or opening (12). The trench or opening (12) can be formed using any suitable means, for example, by a photolithographic etching means. The surface of the semiconductor base material (10), including the trenches or openings (12), is then coated with a suitable dielectric material (13) such as silicon oxide, poly-crystalline silicon, PSG, or BPSG. The dielectric layer (13) will conform to the non-planar surface of the semiconductor base (10), and the trenches and/or openings (12) formed thereupon, to create a non-planar dielectric surface (13) comprising relatively low topographical regions (12′) within the trenches and openings (12), and relatively high topographical regions (14) in the areas in-between the trenches and openings (12). The relatively high topographical regions (14) are known as the “over-active” regions. The relatively low topographical regions (12′) are known as the “over-trench” regions. The surface of the non-planer planar dielectric layer (13) is then planarized, wherein the over-active regions (14) are sufficiently reduced so as to expose the mask layer (11) without exposing the underlying semiconductor base layer (10), thereby leaving the over-trench dielectric layer within the trench or opening (13′). This is accomplished, for example, by polishing the non-planar dielectric layer (13) according to the polishing method of the present invention.

[0012] The present method also can be used to polish a semiconductor device comprising an inter-layer dielectric (ILD). FIGS. 2A and 2B illustrate a typical semiconductor device comprising an interlayer dielectric. For the sake of clarity, well-known features such as doped regions, active devices, epitaxial layers, carrier oxides, as well as other such features have been omitted. As illustrated in FIGS. 2A and 2B, a typical semiconductor device comprising an ILD has a semiconductor base (20) made from any appropriate material, for example, single crystal silicon, gallium arsenide, or other suitable semiconductor materials known in the art. As those of skill in the art will appreciate, the semiconductor base layer (20) can also represent previous layers of interconnects or gate level dielectric layers. Over the semiconductor base (20), a dielectric layer (21) is deposited, which layer typically contains silicon dioxide, PSG, BPSG, silicon nitride, tantalum nitride, titanium nitride, or other suitable dielectric materials known in the art. Metal interconnects (22) are placed over, or embedded within, the dielectric layer (21), for example, by using a photolithography process, thereby creating a metallized layer. Metal interconnects (22) can be made from any suitable metal, for example, aluminum, copper, tungsten, polysilicon, gold, platinum, iridium, ruthenium, alloys, and the like. Generally, a second layer of dielectric material (23) is applied over the metal interconnects and the regions in-between between the metal interconnects (22), resulting in a non-planar dielectric surface (23) that must be polished or planarized before an additional layer of circuitry (e.g., metal interconnects and/or additional dielectric layers) can be applied. The dielectric layer (23) overlying the metal interconnects (22) forms topographically high regions in the surface of the dielectric known as the “stack” dielectric regions (24). The dielectric layer overlying regions in-between the metal interconnects form topographically low regions referred to as the field dielectric regions (25, 26). The difference in height between the relatively low regions (field) (25, 26) and the relatively high regions (stack) (24) of the dielectric is known as the step height. As the dielectric surface is polished (i.e., according to the present inventive method), the step height is reduced. The step height can be measured directly by methods known in the art, such as by using a profilometer. The step height also can be determined by measuring the stack and field thicknesses and calculating the step height using the following formula:

step height=initial step height−(Δstack thickness+Δfield thickness).

[0013] The present method can be used to polish or planarize any part of a semiconductor device, e.g., the metallized (e.g., metal interconnects) (22), dielectric (21, 23), or base layer (20) of a semiconductor device, as described above. The present method can be used, for example, to polish the surface dielectric layer (23) of a semiconductor device with or without breaking through the stack dielectric material to polish the underlying metal interconnects (22), underlying dielectric material (21), or base materials (20). The present method is preferably used to polish the dielectric layer of a semiconductor substrate without breaking through to polishing the underlying metal interconnects. However, if used to polish a metal substrate (e.g., the metal interconnects underlying the dielectric layer), it is preferable that the substrate comprises metals that do not react with the silica abrasive, e.g., a substrate that does not comprise aluminum.

[0014] It is desirable to polish the stack dielectric regions (24), or over-active dielectric regions (14), of a semiconductor substrate with as little loss as possible of the field dielectric (25, 26), or over-trench regions (12′), thereby yielding a high planarization efficiency. Planarization efficiency is the relative difference between the amount of stack dielectric or over-active dielectric removed and the amount of field dielectric or over-trench dielectric removed during the polishing or planarizing process. Planarization efficiency will depend, in part, on the pattern density of the substrate. In the context of ILD applications, the pattern density is the fractional surface area of the substrate surface comprising circuitry (e.g., metal interconnects (22) or stack dielectric regions (24) overlying the metal interconnects (22)). In the context of STI applications, the pattern density is the fractional surface area of the substrate surface comprising trenches or openings (12). Furthermore, the pattern density can vary within a given substrate (“die”). Thus, different areas of the substrate can be polished at different rates (e.g., areas of the ILD having lower pattern densities will be polished at a higher rate than areas having higher pattern densities) giving rise to “within die non-uniformity”(WIDNU) as illustrated by FIG. 2B. Methods of polishing or planarizing a substrate with high planarization efficiency (high selectivity to stack dielectric regions (in the context of ILD) or over-active dielectric regions (in the context of STI)) will tend to reduce WIDNU.

[0015] The present method can be used to polish a semiconductor substrate with a high planarization efficiency. For example, the present method can be used to polish a semiconductor substrate comprising at least 50% pattern density with an efficiency of at least 75%, preferably at least 80%, more preferably at least 90%. This efficiency is, in part, attributed to a high selectivity of the present method to polishing the relatively high topographical regions of the dielectric (e.g., the stack dielectric regions overlaying the metal interconnects in the context of ILD applications, or the over-active dielectric regions in-between the trenches or openings in the context of STI applications) at a faster rate than the relatively low topographical regions of the dielectric (e.g., the regions of dielectric between the metal interconnects in the context of ILD applications, or the regions of dielectric within the trenches in the context of STI applications). The selectivity can be expressed as a ratio of the planarization rate of the stack or over-active dielectric regions to the planarization rate of the field or over-trench dielectric regions. The present inventive method can be used to polish or planarize a substrate with a selectivity ratio of at least about 2:1, preferably at least about 5:1. In preferred modes of practicing the present invention, the selectivity ratio will decrease as planarization continues, thereby exhibiting a “self-stopping” behavior.

[0016] The Polishing Pad

[0017] The polishing pad can comprise materials of varying compositions and hardnesses, provided the polishing pad comprises a polymeric polishing surface capable of communicating with the silica abrasive particles of the polishing slurry. The term “communicating” as it is used in conjunction with the present invention refers to the interaction of the silica abrasive particles with the polymeric polishing surface. While not wishing to be bound by any particular theory, it is thought that, when using a polishing pad and silica abrasives as described herein, the silica abrasive particles associate with, or temporarily adhere to, the surface of the polishing pad. This may be caused, for example, by chemical, physical/mechanical, or hydrogen bonding forces, or a combination of such mechanisms, or even other mechanisms. The silica particles and/or polymeric polishing pad surface may, for instance, become slightly, partially, or wholly softened or dissolved during polishing according to the method described herein. As a result, the silica particles are believed to adhere to the polishing surface of the polishing pad forming an in situ fixed abrasive article, more specifically a self-regenerating in situ fixed abrasive article, which allows planarization of a substrate with the efficiency of a fixed abrasive article while causing significantly fewer defects to form in the substrate surface.

[0018] The polishing pad and/or polishing surface can be woven or non-woven and can comprise any suitable polymer of varying density, hardness, thickness, compressibility, ability to rebound upon compression, and compression modulus. The polishing pad used in conjunction with the present inventive method preferably has a density of about 0.6-0.95 g/cm³, a Shore A hardness rating of less than about 100 (e.g., about 40-90), a thickness of at least about 0.75 mm (e.g., about 0.75-3 mm), compressibility of about 0-10% (by volume), the ability to rebound to at least about 25% (by volume) (e.g., 25-100%) after compression at about 35 kpa, and a compression modulus of at least about 1000 kPa. Examples of suitable polymers include polyurethanes, polymelamines, polyethylenes, polyesters, polysulfones, polyvinyl acetates, polyacrylic acids, polyacrylamides, polyviylchlorides, polyvinylfluorides, polycarbonates, polyamides, polyethers, polystyrenes, polypropylenes, nylons, fluorinated hydrocarbons, and the like, and mixtures, copolymers, and grafts thereof Preferably, the polishing pad comprises a polyurethane polishing surface. The polishing pad and/or surface can be formed from such materials using suitable techniques recognized in the art, for example, using thermal sintering techniques. Furthermore, the polishing pad formed from such materials may be substantially porous (i.e, having open or closed pores) or substantially non-porous. Porous pads preferably have a pore diameter of about 1-1000 μm and a pore volume of about 15-70%. The polishing pad and/or surface also can be perforated or unperforated to any degree. Preferably, the polishing pad comprises a perforated polishing surface.

[0019] The polishing surface of the polishing pad preferably comprises a multiplicity of cavities which can include and/or be in addition to any pores or perforations as previously described. Without wishing to be bound by any particular theory, the multiplicity of cavities are thought to provide channels through which the polishing slurry can flow. Cavities, for the purpose of the present invention, includes recesses or indentations in the surface of the pad, protrusions arranged in such fashion as to form recesses between the protruding portions of the surface of the pad, or any combination of recesses and protrusions. The recesses or protrusions can be any suitable size or shape. The multiplicity of cavities form a macro-texture on the polishing surface of the polishing pad, which can further include a micro-texture imposed upon the recessed and/or protruding portions of the macro-texture. The multiplicity of cavities forming the macro-texture and/or micro-texture can have any dimension and arrangement. The cavities can, for example, be arranged randomly or as a pattern. Preferably, the cavities are arranged in a non-circular or non-spiral arrangement. For example, it is preferred that the cavities are arranged so as not to form trenches or grooves over a substantial portion of the polishing surface of the pad. Thus, preferably at least about 50% (e.g., about 50%-75%) o even at least about 80% (e.g., about 90%-100%) of the pad surface should be essentially free of concentric or spiral grooves or trenches. The pad can, for example, be grooved to the extent required to facilitate removal of the semiconductor substrate from the polishing pad and/or platen, as will be appreciated by those of ordinary skill in the art.

[0020] The polishing pad optionally comprises a backing. The backing portion can comprise any suitable backing material known in the art. The backing can, for example, be flexible or rigid in varying degrees, as will be appreciated by those of ordinary skill in the art. Typical backing materials, for example, include polymeric films, metal foils, cloth, paper, vulcanized fiber, and combinations thereof.

[0021] The polishing pad used in conjunction with the present method should be substantially free of bound abrasive particles. The term “bound abrasive particles,” as used in the context of this invention, refers to abrasive particles that are affixed to the polishing surface of the polishing pad by way of an adhesive, binder, ceramer, resin, or the like. The term “bound abrasive particles” also refers to abrasives that have been impregnated within a polishing pad so as to form an integral part of the polishing pad, such as, for example, a fibrous batt impregnated with an abrasive-containing polyurethane dispersion. Silica particles that become associated with the polishing surface of a polishing pad during polishing according to the methods described herein, i.e., by communicating with the polishing surface of the polishing pad as described previously, are not considered to be “bound abrasive particles” in the context of the present invention.

[0022] The Polishing Slurry

[0023] The polishing slurry for use in conjunction with the present method comprises a liquid carrier and silica particles as an abrasive. Suitable silica particles are commercially available and can be prepared by known methods, for example, by wet chemical methods such as condensation-polymerization or colloidal precipitation. In preferred preparations, the silica particles have an average particle size of less than 30 nm, preferably having an average particle size of about 25 nm or less, more preferably about 20 nm or less. Preferred preparations contain silica particles about 90% or more of which (by number) have a particle size less than 30 nm (e.g., a particle size of about 20 nm or less). Preferably, the abrasive particles are such that at least about 95%, 98%, or even substantially all (or actually all) of the abrasive particles (by number) have a particle size less than 30 nm (e.g., a particle size of about 20 nm or less). These particle size preferences for the abrasive particles (i.e., whereby at least about 90%, 95%, 98%, substantially all, and all of the abrasive particles (by number) are less than or equal to a specific size of abrasive particle) also can pertain to other particle sizes, such as 25 nm, 20 nm, 15 nm, and 10 nm.

[0024] Similarly, the abrasive particles of the composition of the present invention can be such that at least about 90%, 95%, 98%, or even substantially all (or actually all) of the abrasive particles (by number) have a particle size no less than l nm. These particle size preferences for the abrasive particles (i.e., whereby at least about 90%, 95%, 98%, substantially all, and all of the abrasive particles (by number) are no less than a specific size of abrasive particle) also can pertain to other particle sizes, such as 7 nm, 10 nm, 15 nm, and 25 nm.

[0025] The percentage values used herein to describe the nature of the abrasive particles in terms of particle size are percentages “by number,” rather than being weight percentages, unless otherwise noted. The particle size of the abrasive particles refers to the particle diameter. The particle size can be measured by any suitable technique. The particle size values set forth herein are based on a visual inspection, specifically by way of transmission electron micrography (TEM), of a statistically significant sample of the abrasive particles, preferably at least 200 particles.

[0026] The particle size distribution of abrasive particles can be characterized by geometric standard deviation by number, referred to as sigma-g (σg). The σg values can be obtained by dividing (a) the diameter at which 84% of the abrasive particles (by number) are less than by (b) the diameter at which 16% of the abrasive particles (by number) are less than (i.e., σ_(g)-d₈₄/d₁₆). Monodispersed abrasive particles have a σ_(g) value of about 1. As the abrasive particles become polydispersed (i.e., include particles of increasingly different size), the σ_(g) value of the abrasives particles increases above 1. The abrasive particles typically have a σ_(g) value of about 2.5 or less (e.g., about 2.3 or less). The abrasive particles desirably have a σ_(g) value of at least about 1.1 (e.g., about 1.1-2.3 or even about 1.1-1.3), preferably a σ_(g) value of at least about 1.3 (e.g., about 1.5-2.3 or even about 1.8-2.3).

[0027] The polishing slurry typically will contain from about 0.5-50 wt. % of the silica abrasive particles. Preferred formulations comprise about 10-40 wt. % silica, more preferably about 20-35 wt. % silica (e.g., about 30-35 wt. % silica).

[0028] The liquid carrier of the polishing slurry can comprise any suitable liquid; however, preferred preparations of the polishing slurry comprise an aqueous liquid carrier. The liquid carrier also can contain various additives known in the art. As those of ordinary skill in the art will appreciate, the additives used in the polishing slurry will vary depending on the application in which the polishing slurry will be used. Such additives include surfactants (e.g., cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants, fluorinated surfactants, and mixtures thereof), polymeric stabilizers or other surface active dispersing agents (e.g., phosphoric acid, organic acids, tin oxides, and phosphonate compounds), pH buffers (e.g., potassium phosphate), and polishing accelerators such as catalysts, oxidizers, and chelating or complexing agents (e.g., metal, particularly ferric, nitrates, sulfates, halides (including fluorides, chlorides, bromides, and iodides), compounds with carboxylate, hydroxyl, sulfonic, and/or phosphonic groups, di-, tri-, multi-, and poly-carboxylic acids and salts (such as tartaric acids and tartrates, malic acid and malates, malonic acid and malonates, gluconic acid and gluconates, citric acid and citrates, phthalic acid and phthalates, pyrocatecol, pyrogallol, gallic acid and gallates, tannic acid and tannates), amine-containing compounds (such as primary, secondary, tertiary, and quaternary amines and amino acids), peroxides, periodic acid and salts, perbromic acid and salts, perchloric acid and salts, perboric acid and salts, iodic acid and salts, permaganates, potassium ferricyanide, chlorates, percarbonates, persulfates, bromates, chromates, cerium compounds, and mixtures thereof). However, preferred preparations of the polishing slurry consist essentially of silica particles and water.

[0029] The pH of the polishing slurry can be any suitable pH. The pH may affect performance and, therefore, generally is selected based upon the nature of the substrate being polished, including the chemical composition and topography of the substrate. In some cases, e.g., where the substrate contains metal oxide or (e.g., silicon dioxide) or metal nitride (e.g., titanium nitride), the polishing slurry preferably has a pH greater than 7. In some instances, the pH of the polishing slurry is about 7-13, preferably about 8.5-12, especially about 9 to 11. The pH of the polishing slurry can be adjusted using any suitable pH adjusting agent. Examples of suitable agents include hydroxide compounds such as potassium hydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and basic compounds such as amines and the like. The pH adjusting agent can be a mixture of compounds, such as a mixture of potassium hydroxide and lithium hydroxide. The pH adjusting agents can be in the form of a solution, e.g., an aqueous solution. An example of a metal hydroxide-containing solution is a solution of potassium hydroxide in deionized or distilled water in which the potassium hydroxide concentration is about 0.1-0.5 wt. % (e.g., about 0.2-0.3 wt. %).

[0030] Process Conditions

[0031] The present method can further comprise a conditioning step wherein the polishing surface of the polishing pad is cleaned. Such conditioning can be used to control the rate of silica buildup on the pad and, thereby, control the rate of planarization and planarization efficiency achieved using the present inventive method. The polishing surface can be conditioned according to techniques commonly known to those of skill in the art. For example, the polishing surface can be contacted with a conditioning brush, tool, or pad. Preferably, a conditioning tool or pad, for example, a diamond grit conditioning tool or pad, having a coarseness (grit) number of 100 or finer, is used to condition the polishing surface.

[0032] The polishing surface can be conditioned during the polishing or planarization of the substrate (in situ conditioning), or the polishing surface can be conditioned before and/or after polishing the substrate (ex situ conditioning), for example, by conditioning between the polishing of two semiconductor devices. Preferably, the polishing surface is conditioned before and/or after the polishing of the semiconductor device, and is not conditioned during the polishing step.

[0033] In general, there can be numerous planarization steps for a single substrate. For example, a substrate can be wholly or partially planarized (e.g., to remove insulating layers of a semiconductor device without polishing underlying metal interconnects) using the polishing methods disclosed herein, and finished or buffed by other methods. Any suitable polishing pad and polishing slurry can be used to buff the polished substrate. Such polishing pads and polishing slurries are commonly known in the art. When using the present inventive method in conjunction with a finishing or buffing step, a relatively high removal rate of the insulating layers is preferred. If the insulating layer is a metal oxide such as silicon dioxide, the removal rate should be at least 100 Angstroms per minute, preferably at least 500 Angstroms per minute, more preferably at least 1000 Angstroms per minute, and most preferably at least 1500 Angstroms per minute. In some instances, it may be desirable for the removal rate to be as high as at least 2000 Angstroms per minute, and even at least 3000 or 4000 Angstroms per minute. If the material being removed during planarization is harder and/or more chemically durable than silica, then the removal rates may be lower. Conversely, if the material being removed during planarization is softer than silica, then the removal rates may be higher. However, although it is generally desirable to have a high removal rate, the removal rate must be selected such that it does not compromise the desired topography of the wafer surface.

[0034] The interface pressure between the polishing pad and semiconductor wafer (i.e., the contact pressure) is any suitable pressure that eliminates or reduces dishing and/or non-uniformity in the surface of the substrate. The interface pressure is preferably less than about 150 kPa, more preferably less than about 100 kPa, even more preferably less than about 60 kPa, and most preferably less than 40 kPa and even as low as 20 kPa or less. Also, two or more processing conditions within a planarization process may be used. For example, a first processing segment may comprise a higher interface pressure than a second processing segment. The interface pressure may be a combination of downforce between the polishing pad (platen) and the substrate, and backpressure exerted behind the substrate. Typically, between about 10-70 kPa of each of downforce pressure and backpressure will be used to eliminate or reduce dishing and/or non-uniformity in the substrate surface.

[0035] Any suitable platen speed can be used to polish according to the present method. The platen can rotate about 5-10,000 rpm, typically about 10-1000 rpm, preferably about 20-300 rpm, more preferably about 30-150 rpm. The substrate holder (carrier) also may rotate in the same or opposite direction as the platen and in any circular or non-circular manner. The carrier generally rotates about 5-300 rpm, preferably about 10-100 rpm, more preferably about 20-50 rpm.

[0036] The following examples further illustrate the present invention but, of course, should not be construed as in any way limiting its scope.

EXAMPLE 1

[0037] This example demonstrates a method of polishing the ILD of a semiconductor device according to the present invention. Two semiconductor devices (Device Nos. 1A and 1B) comprising ILD test patterns (MIT design mask) having systematically varied pattern densities ranging from 8-100%, a 250 micron pitch, and an initial step height of 9000 Å were polished on an IPEC 472 polishing machine using a down force of 51.75 kPa, a back pressure of 20.7 kPa, a platen speed of 37 rpm, a carrier speed of 24 rpm, and a slurry flow rate of 220 ml/min. A perforated polyurethane polishing pad was used comprising a polishing surface having a multiplicity of cavities arranged in a non-circular pattern and a single groove to facilitate removal of the semiconductor substrate from the platen after polishing. Only ex situ conditioning was performed.

[0038] The first semiconductor device (Device No.1 A) was polished using a commercially available fumed silica slurry (Cabot Semi-Sperse® 12 (12% solids)) having an average particle size (by number) of 90 nm. Measurements of the step height were taken in regions of the semiconductor device having 52% pattern density. After 90 seconds, the semiconductor device was polished to about 2000 Angstroms with about 700 Angstroms of field loss. After 150 seconds, the semiconductor device was polished to a step height of about 500 Angstroms with about 2400 Angstroms of field loss.

[0039] The second semiconductor device (Device No. 1B) was polished in the same manner, except that the polishing slurry used was prepared by diluting an ammonia stabilized precipitated colloidal silica suspension having an average particle size (by number) of about 20 nm (Nyacol® 20NH40 manufactured by Akzo Nobel) with deionized water to a solids concentration of 30 wt. % and adjusting the pH to about 10.5 with KOH, in accordance with the present invention. Measurements of the step height were taken in regions of the semiconductor device having 52% pattern density. After 90 seconds, the semiconductor device was polished to a step height of about 3000 Angstroms with about 250 Angstroms of field loss. After 150 seconds, the semiconductor device was polished to a step height of 500 Angstroms with about 700 Angstroms of field loss.

[0040] This example demonstrates the effectiveness and efficiency of the present method of polishing a substrate in polishing a semiconductor device as compared to conventional methods. The device polished according to the present method (Device No. 1B) was polished to approximately the same step height in about the same amount of time as the device polished according to the conventional method (Device No. 1A), but with greater than a 3-fold reduction in field-loss.

EXAMPLE 2

[0041] This example demonstrates a method of polishing a semiconductor device according to the present invention. Two semiconductor devices (Devices Nos. 2A and 2B) having the same characteristics as those described in Example 1 were polished in the same manner as described in Example 1. Device 2A was polished using a conventional 90 nm average particle size fumed silica slurry. Device 2B was polished using a polishing slurry prepared by diluting an ammonia stabilized precipitated colloidal silica suspension having an average particle size of about 20 nm (Nyacol® 20NH40 manufactured by Akzo Nobel) with deionized water to a solids concentration of 30 wt. % and adjusting the pH to about 10.5 with KOH, in accordance with the present invention.

[0042] The step height was measured after polishing for 60 seconds, 90 seconds, 120 seconds, and 150 seconds in two regions of each semiconductor device: the array field (regions of 52% pattern density) and the open field (regions of 8% pattern density). The step height was plotted against time to yield a planarization rate curve for each substrate in each region. By extrapolation,the time at which the substrate would have been 95% planarized was determined. The planarization efficiency (sp) at each polishing interval also was calculated according to the following equation:

εp=1−(Δ field thickness/Δ stack thickness).

[0043] The calculated planarization efficiency was plotted against time to yield a planarization efficiency curve for each substrate in each region. The planarization efficiency for each substrate at each region at the time 95% planarization would have been achieved, as identified using the planarization rate curve, is set forth in Table 1. TABLE 1 Open Field Array Field Substrate Efficiency Efficiency Device No. 2A 49.7% 81.1% Device No. 2B 68.7% 90.1%

[0044] As is apparent from the data recited in Table 1, the method of the present invention can be used to polish a semiconductor device with improved planarization efficiency in both array and open field dielectric regions.

EXAMPLE 3

[0045] This example demonstrates the effect of conditioning on the present method of polishing. A semiconductor device having the same characteristics as the devices described in Example 1 was polished in the same manner as Device No. 1B of Example 1, except that the polishing surface of the polishing pad was conditioned in situ using a diamond conditioning pad. Step height measurements were taken in regions of the semiconductor device having 32% pattern densities. After about 150 seconds, the semiconductor device was polished to a step height of about 0 Angstroms with about 1200 Angstroms of field loss. Another similar semiconductor device was polished according to the same method without conditioning (ex situ conditioning)to a step height of about 0 Angstroms with about 750 Angstroms of field loss. Using ex situ conditioning in conjunction with the present invention reduced field loss by about 37%.

EXAMPLE 4

[0046] This example demonstrates the effect that concentric grooves formed upon a substantial portion of the polishing surface of the polishing pad have on the polishing rate. Two semiconductor devices were polished in the same manner as Device No. 1B of Example 1. The first device (Device No. 4A) was polished using a perforated polyurethane polishing pad comprising a polishing surface having concentric grooves formed in the surface of the pad (Mirra™ IC-1000 K-groove available from Applied Materials). The second device (Device No. 4B) was polished using a polishing pad that was the same in all respects, except that the polishing surface of the pad had a multiplicity of cavities arranged in a non-circular pattern and only a single groove to facilitate removal of the semiconductor substrate from the platen after polishing (Mirra™ IC-1000 available from Applied Materials). The step height was measured in regions of the semiconductor devices having 52% pattern density at 0 seconds, 90 seconds, 120 seconds, 1 50 seconds, and 210 seconds. The results are set forth in Table 2. TABLE 2 Step Height (Å)/Time 90 120 150 210 Device 0 seconds seconds seconds seconds seconds 4A 9000 7200 6800 6500 6100 4B 9000 4500 2700 1900 1700

[0047] The step height was plotted against time for each device to yield the planarization rate curves depicted in FIG. 3. As illustrated in FIG. 3, the polishing rate using the perforated pad (Device 4B) is initially greater than the rate using the grooved pad. However, as polishing continues, the polishing rate using the perforated pad slows as the substrate becomes more planar, whereas the polishing rate using the grooved pad (Device 4A) continues at approximately the same rate over time. This example demonstrates the desirable “self-stopping” behavior of a preferred mode of practicing the present invention.

[0048] All of the references cited herein, including patents, patent applications, and publications, are hereby incorporated in their entireties by reference.

[0049] While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations of the preferred embodiments may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the following claims. 

What is claimed is:
 1. A method of polishing a substrate, which method comprises (a) providing a polishing slurry comprising a liquid carrier and silica particles, wherein the average size of the silica particles is less than 30 nm, (b) providing a polymeric polishing pad substantially free of bound abrasive particles and having a polishing surface comprising a multiplicity of cavities, and (c) polishing the surface of the substrate by contacting the polishing slurry and the polishing pad with the substrate and moving the polishing pad relative to the substrate.
 2. The method of claim 1, wherein the polishing surface is polyurethane.
 3. The method of claim 1, wherein about 90% or more of the abrasive particles (by number) have a particle size less than about 30 nm.
 4. The method of claim 1, wherein the silica particles have an average particle size of about 20 nm or less.
 5. The method of claim 1, wherein about 90% or more of the abrasive particles (by number) have a particle size of about 20 mn or less.
 6. The method of claim 1, wherein the silica particles are condensation-polymerized silica particles.
 7. The method of claim 1, wherein the polishing slurry consists essentially of silica particles and water.
 8. The method of claim 1, wherein the polishing slurry is basic.
 9. The method of claim 8, wherein the polishing slurry has a pH of about 8.5-12.
 10. The method of claim 1, wherein the substrate is a semiconductor device.
 11. The method of claim 1, wherein the substrate is a dielectric layer of a semiconductor device.
 12. The method of claim 10, wherein the polishing pad is not conditioned during the polishing of the semiconductor device.
 13. The method of claim 12, wherein the polishing pad is conditioned before and/or after polishing the semiconductor device.
 14. The method of claim 10, wherein the semiconductor device comprises device geometries below about 0.25 μm.
 15. The method of claim 1, wherein the silica particles communicate with the polishing surface to form an in situ fixed abrasive.
 16. The method of claim 1, wherein the silica particles and the pad communicate in a manner consistent with a self-regenerating fixed abrasive article. 